Method of manufacturing a flexible tubular member by coiling a shaped metallic strip about a tube of snythetic plastic material

ABSTRACT

Method of manufacturing a flexible tubular member by coiling a shaped metallic strip about a tube of synthetic plastic material in which the tube of synthetic material is flattened and curved into an arcuate cross-section before entering the forming head which forms the metallic strip into a tube. The invention includes apparatus for carrying out this process.

nited States Mazuir et al.

atent [54] METHOD OF MANUFACTURING A FLEXIBLE TUBULAR MEMBER BY COILINGA SHAPED METALLIC STRIP ABOUT A TUBE OF SNYTHETIC PLASTIC MATERIAL July10, 1969 France ..6923533 21 Appl. No.2 52,296

[52] US. Cl ..29/456, 29/234, 93/59 MT, 93 59 PL/, 93/94 M [51] Int. Cl...B23q 3/154, B23q 3/18, B3lc 13/00 [58] Field of Search..29/234, 456;93/59 PL, 59 MT, 93/77 LL, 80, 94 M 1 Aug. 29, 1972 [56] ReferencesCited UNITED STATES PATENTS 3,580,461 5/ 197 l Dobell ..93/94 M PrimaryExaminer-Wayne A. Morse, Jr. Attorney-H0lcombe, Wetherill & Brisebois [57] ABSTRACT Method of manufacturing a flexible tubular member by coilinga shaped metallic strip about a tube of synthetic plastic material inwhich the tube of synthetic material is flattened and curved into anarcuate cross-section before entering the forming head which forms themetallic strip into a tube. The invention includes apparatus forcarrying out this process.

9 Claims, 4 Drawing Figures S A 2 i METHOD OF MANUFACTURING A FLEXIBLETUBULAR MEMBER BY COILING A SHAPED METALLIC STRIP ABOUT A TUBE OFSNYTHETIC PLASTIC MATERIAL SUMMARY OF THE INVENTION This inventionrelates to a new process of manufacturing a flexible tubular member byhelically winding a shaped metallic strip about a flexible tube made ofa synthetic material.

The present invention also relates to the articles produced by thisprocess and to a machine for carrying out the process.

The invention relates more particularly to the manufacture of flexibletubes used in the drilling of oil wells. The tubes used for this purposeare equipped at their lower ends with a drilling bit and areprogressively introduced into the bore which they make in the ground.

These flexible drilling pipes generally comprise an inner liningconsisting of a tube of synthetic material which is encircled by aflexible tubular member made by helically winding a shaped metallicstrip. This helical member is itself encircled by several layers ofwires or strong shaped metallic strips, which are applied to itsperiphery in accordance with a technique analogous to the one employedin the manufacture of cables.

It is the principal object of the present invention to solve oneparticular problem which is posed by the passage of the tube ofsynthetic material through the forming head which forms the shapedmetallic strip into a helix so that it may constitute the tubularflexible member.

This problem results from the fact that the tubeof synthetic materialmust have an outer diameter equal to, and sometimes even slightlygreater than, the inner diameter of the flexible tubular member which isto manufactured around it. In order to helically deform the metallicstrip, it is essential, because of the elasticity of the strip, todeform it by passing it through a point which is located substantiallyinside the circle which defines the section of the tubular member whichis being made.

It accordingly follows that the strip which is being deformed into ahelix resists the normal passage of the synthetic tube which must bepositioned inside the tubular member.

The present invention relates to a process which offer a solution tothis problem.

It is an object of the present invention to provide a process formanufacturing a flexible tubular member from a shaped metallic stripcoiled about a synthetic tube, characterized by the fact that the tubeof synthetic material is brought into alignment with the flexibletubular member being formed, the synthetic tube is then substantiallyflattened and passed through at least one ring, which is preferablyrotated, and has a diameter less than the internal diameter of thetubular member being formed, whereas the flattened tube of syntheticmaterial assumes within said ring a generally arcuate form. The tubeformed in this manner passed inside the forming head which helicallycoils the strip from which the tubular member is being formed, and thetube of synthetic material is then returned to its original cylindricalform inside the tubular member which has just been completed by an ovoidmember inside the tube of synthetic material. This ovoid member has anexternal diameter approximating the internal diameter of the tube offlexible material and is kept in place by suitable means, such as thepressure created by fluid inside part of the tube, or magnetic meansacting on the ovoid member from outside the tubular member.

In accordance with a preferred embodiment of the invention, the ringsthrough which the tube of synthetic material passes before it enters theforming head are rotatably driven at the same speed as the tube ofsynthetic material and the tubular member being formed.

In said preferred embodiment of the invention, the ring near the forminghead is substantially tangent to a generatrix delimiting the inside ofthe tubular member being formed so as to separate the opposite side ofthe tube of synthetic material from the deforming member which imposesthe helical defonnation on the shaped strip.

In one particular form of this preferred embodiment several successiveopenings having increasing diameters are used.

In a first method of carrying out the invention an ovoid member is used,which is positioned downstream of the forming head, and is kept in thisposition by means of a gas under pressure in the downstream part of thetubular member. In another method the position of the ovoid member ismaintained by magnetic means acting through the wall of the tube beingformed. In this embodiment the tube of synthetic material is advancedwhile flattened by a partial vacuum maintained upstream of the tube.

In a second method of carrying out the process according to theinvention second ovoid member is placed inside the tube upstream of thering or rings.

In this case, the two ovoid members are connected by a duct whichterminates at the ends of the ovoid members, thus creating a continuouscommunication between the upstrem and downstream sections of the tube ofsynthetic material. The duct which connects the two ovoid memberscomprises a Venturi so that when fluid is circulated through this ductit causes a pressure drop between the ovoid members which flattens thetube during its passage through the ring or rings. The circulation offluid in the tube which connects the two ovoid members may be easilyproduced by creating, for example a pressure downstream of the ovoidmembers which is greater than the pressure upstream of these ovoidmembers. It follows that the passage of the tube of synthetic materialthrough the forming head is thus readily facilitated, while the tube isreturned to a cylindrical state by internal pressure in the tubedownstream of the ovoid members.

According to a method of carrying out the invention, two ovoid membersare used, which are placed upstream and downstream of the ring or ringspreceding the forming head. In this third embodiment of the inventionthe ovoid member upstream of the forming head is held in position bymagnetic means outside the tube of synthetic material and the two ovoidmembers are connected to each other by a rod or cable. The ovoid memberwhich is situated downstream is pierced by an orifice which insures thatthere is equal pressure upstream and downstream of this second ovoid.The tube of synthetic material is subjected to internal pressureupstream of the ovoid members, whereas downstream it is only subjectedto atmospheric pressure, just as in the part between the two ovoidmembers. In this case, it is the simple passage of the tube through thering or rings which results in its compression to an arcuate form topermit its passage through the forming head, whereas the ovoid which islocated downstream returns the tube of synthetic material to itsoriginal cylindrical shape.

It is a further object of the present invention to provide, as a newarticle of manufacture, a device for carrying out the above-describedprocess. This apparatus is characterized by the fact that it comprisesin combination: means for advancing a tube of synthetic material alongthe axis of a cylindrical member being formed by helically winding ashaped metallic strip, at least one ring preferably rotated, and havingan inner diameter less than the inner diameter of the tubular memberbeing manufactured, and an ovoid member having an outer diametersubstantially equal to the inner diameter of the tube of syntheticmaterial, mounted downstream of the forming head which forms theflexible tubular member, said ovoid member being kept in position byfluid pressure, or by magnetic means.

In yet another embodiment, the apparatus comprises a second ovoid memberpositioned upstream of the ring, the two ovoid members being connectedby a duct which opens on opposite sides of the two ovoids and whichcomprises a Venturi between the two ovoids which makes it possible tocreate a partial vacuum by passing a fluid through said duct.

In another embodiment, the device also comprises two ovoids mountedupstream and downstream of the ring or rings and of the forming head,the two ovoid members being connected to each other by a rod or cable,the upstream ovoid being in substantially fluidtight contact with theupstream wall of the tube and being kept in place by a magnetic device,whereas the downstream ovoid is pierced by an opening which permits thesame pressure, for example atmospheric pressure, to be maintained onboth sides of said downstream ovoid.

It is a further object of the present invention to provide flexibletubular members made by helically winding a shaped metallic strip andcomprising an inner sheath consisting of a tube of synthetic material,manufactured by the foregoing process, utilizing the apparatus which hasjust been described.

In order that the invention may be clearly understood, severalembodiments thereof will now be described, purely by way of illustrationand example, with reference to the accompanying drawing, on which:

FIG. 1 is a schematic perspective view showing apparatus for carryingout the process according to the invention;

FIG. 2 is a schematic axial sectional view of the device ofFIG. 1;

FIGS. 3 and 4 are axial sectional views of a modified device for use incarrying out the second and third methods of practicing the invention.

FIG. 1 shows how the shaped metallic strip 1, which is advanced in thedirection of the arrow, is helically shaped by the forming fingers 2, 3and 4, which are attached to a forming head 5, which is schematicallyshown. The strip 1 is thus transformed into a cylindrical member 6.

As shown clearly in FIG. 2, the end of the forming finger 2 extendssubstantially inside the circular section of the tubular member 6. Itfollows that the presence of this forming finger, which is indispensableto the helical winding of the strip 1, impedes the passage of the tube 7of synthetic material when this is not deformed and has an externaldiameter equal or even slightly greater than the internal diameter ofthe tubular member 6.

In accordance with the invention, the tube 7 is passed, before ittravels through the forming head 5, through the rings 8, 9 and 10, whichhave a diameter less than that of the tubular member 6, the section ofthe last ring 10 being less than that of the open space in the tubularbody being formed by the forming fingers 2, 3 and 4.

The rings 8, 9 and 10 are mounted to rotate and are driven by a shaft 11through the chains 12, which are schematically shown, at a speed ofrotation which corresponds to the speed of rotation of the tubularmember 6 being formed.

The tube 7 of synthetic material is itself rotated at the same speed byfeeding means, not shown.

As may be seen in particular on FIG. 2, the rings 8, 9 and 10 havediameters which decrease successively. The ring 10 lies opposite theopen end of the tubular member 6 at the level of the forming head, thelower edge of the tube 7 being substantially at the level of the loweredge of the tubular member 6. While passing through the rings 8, 9 and10, the tube of synthetic material 7, which is initially flattened, atthe left hand side of FIGS. 1 and 2, is curved into an arcuate sectionas it passes through these rings.

The tube 7 is returned to its cylindrical shape downstream of theforming head 5 by means of an ovoid member 13, the diameter of which issubstantially equal to the inner diameter of the tube 7. This ovoidmember may carry sealing rings 14 which insure a fluid-tight contactbetween the ovoid and the inner surface of the tube 7.

The ovoid 13 may be kept in position only by the fact that a pressuresumcient to resist the drawing action of the tube 7 is maintaineddownstream of the ovoid, for example at the point 15 of the tube 7.

In an alternative arrangement, the position of the ovoid may bemaintained by providing permanent magnets 16 around the periphery of theovoid 13, which magnets cooperate with pole pieces or electromagneticmembers outside the tubular member 6 to keep the ovoid 13 in position.It will be seen that, due to the simple arrangement which has just beendescribed, it is possible to form the tubular member 6 about a tube ofsynthetic material 7 having the same diameter.

FIG. 3 shows a variation in which the tube of synthetic material isadvanced from the left in a cylindrical form while being maintainedunder pressure by a pressurized fluid, such as air.

FIG. 3 shows all the components which have been described with referenceto FIGS. 1 and 2 but, in this case, the device also comprises a secondovoid 17 which is mounted upstream of the ring 8 and which is alsoprovided with permanent magnet 16 cooperating with pole pieces (notshown), and a sealing ring 14. Moreover the ovoid 13 has a centralorifice 13a whereas the ovoid 17 has a central orifice 17a. These twoorifices are in communication through a duct 18 which is equipped with aVenturi 19 making it possible to provide a partial vacuum in the zonebetween the ovoids 13 and 17.

In the embodiment illustrated, the two ovoids are also connected by acable 19 which insures the rigidity of the connection between the olives13 and 17, but it follows that the cable 19 may be eliminated when theduct 18 is made sufficiently strong.

In order to utilize the device of FIG. 3 it suffices to introduce airunder pressure into one of the two ends of the tube 7, for example theend 15 in the case of FIG. 3, and provide for the release of air on theopposite side of the tube, so that there is a constant circulation ofair at high speed in the duct 18 to produce by Venturi effect a partialvacuum inside the two tubes 7 between the two ovoids l3 and 17. Thispartial vacuum results in flattening of the tube, which then passeseasily through the rings 8, 9 and which have, as in the cases of FIGS. 1and 2, diameters which decrease progressively.

By way of example, satisfactory results may be obtained by providing atpoint 15, that is to say downstream of the tube 7, a pressure of 4atmospheres and adjusting the escape of air from the upstream end of thetube 7 so as to have a pressure there of about 2 atmospheres.

In this case as well, the assembly consisting of the ovoids l3 and 17,the tube 7 and the rings 8, 9 and 10 rotates at the same time as thetubular member 6 and at the same speed.

FIG. 4 shows a third method of carrying out the process according to theinvention, the principal components of the device having already beenshown on FIGS. 1, 2 and 3. In the case of the embodiment of FIG. 4 theovoids 13 and 17 are connected by a rigid bar 20 and only the ovoid 13is provided with an orifice 13a which passes all the way through it. Theovoid 17 is preferably provided with a sealing ring 14 and permanentmagnet 16 which cooperate with magnetic member 16a situated outside thesynthetic tube 7. In this case the magnetic members 16a have beenschematically shown in broken lines, because they are indispensable tothis embodiment of the process.

The tube of synthetic material 7 is fed from the left while being keptunder slight pressure so that it remains in a cylindrical state and isfolded on itself by passing through the rings 8, 9 and 10, which reduceit to an arcuate section to pass through the forming head 5.

The ovoid 13, which is attached to the ovoid 17 by the rod 20, returnsthe tube 7 to its cylindrical form.

In this embodiment of the invention, the orifice 13a in the ovoid 13serves to keep the part of the tube 7 between the two ovoids atatmospheric pressure. The part of the tube downstream of the ovoid 13 isalso at atmospheric pressure, so as to evacuate downstream leakages ofair through the ovoid 17.

It will of course be appreciated that the embodiments which have justbeen described have been given purely by way of example and may bemodified as to detail without thereby departing from the basicprinciples of the invention. In particular, it is obvious that it ispossible to carry out the invention using a different number of rings orwith rings which are not rotated, with the tube 7 sliding while turninginside these stationary rings. Moreover, various components of theapparatus may be made in a different manner from those illustrated.

What is claimed is:

1. In a process for manufacturing a flexible tubular member by helicallycoiling a shaped metallic strip in a forming head about a cylindricaltube of synthetic material, the improvement according to which saidprocess comprises the steps of bringing the tube of synthetic materialinto alignment with the tubular member being formed, at least partiallyflattening the tube of synthetic material, and passing it through atleast one orifice having a curved periphery and a diameter less than theinternal diameter of the tubular member being manufactured, whereby thetube of flattened synthetic material is shaped within said orifice tohave an arcuate cross-section, passing said tube through the forminghead, and restoring said tube of synthetic material to its initialcylindrical shape inside the tubular member which has been formed bypassing it around an ovoid within said synthetic tube downstream of saidforming head having an external diameter substantially equal to theinner diameter of said tube, while preventing axial movement of saidovoid in said tube by exerting fluid pressure against the downstreamside of said ovoid.

2. In a process for manufacturing a flexible tubular member by helicallycoiling a shaped metallic strip in a forming head about a cylindricaltube of syntheticmaterial, the improvement according to which saidprocess comprises the steps of bringing the tube of synthetic materialinto alignment with the tubular member being formed, at least partiallyflattening the tube of synthetic material, and passing it through atleast one orifice having a curved periphery and a diameter less than theinternal diameter of the tubular member being manufactured, whereby thetube of flattened synthetic material is shaped within said orifice tohave an arcuate cross-section, passing said tube through the forminghead, and restoring said tube of synthetic material to its initialcylindrical shape inside the tubular member which has been formed bypassing it around an ovoid within said synthetic tube downstream of saidforming head, said ovoid having magnetic properties and an externaldiameter substantially equal to the inner diameter of said tube, whilepreventing movement of said ovoid axially of said tube by subjecting itto a magnetic field.

3. In a process for manufacturing a flexible tubular member by helicallycoiling a shaped metallic strip in a forming head about a cylindricaltube of synthetic material, the improvement according to which saidprocess comprises the steps of bringing the tube of synthetic materialinto alignment with the tubular member being formed, by passing saidtube of synthetic material over a first ovoid having a diametersubstantially equal to the diameter of said tube, at least partiallyflattening the tube of synthetic material, and passing it through atleast one orifice having a curved periphery and a diameter less than theinternal diameter of the tubular member being manufactured, whereby thetube of flattened synthetic material is shaped within said orifice tohave an arcuate cross-section, passing said tube through the forminghead, and restoring said tube of synthetic material to its initialcylindrical shape inside the tubular member which has been formed bypassing it around a second ovoid within said synthetic tube positioneddownstream of said forming head and having an external diametersubstantially equal to the inner diameter of said tube, said secondovoid being connected to said first ovoid by a duct provided with aVenturi opening into said tube between said ovoids, and said tube beingflattened by passing a fluid through said duct to cause said Venturi toreduce the pressure in said tube.

4. Process as claimed in claim 3 which comprises the step of utilizing amagnetic field to hold at least one of said ovoids in position.

5. In a process for manufacturing a flexible tubular member by helicallycoiling a shaped metallic strip in a forming head about a cylindricaltube of synthetic material, the improvement according to which saidprocess comprises the steps of bringing the tube of synthetic materialinto alignment with the tubular member being formed, by passing saidtube of synthetic material over a first ovoid having a diametersubstantially equal to the diameter of said tube, at least partiallyflattening the tube of synthetic material, and passing it through atleast one orifice having a curved periphery and a diameter less than theinternal diameter of the tubular member being manufactured, whereby thetube of flattened synthetic material is shaped within said orifice tohave an arcuate cross-section, passing said tube through the forminghead, and restoring said tube of synthetic material to its initialcylindrical shape inside the tubular member which has been formed bypassing it around a second ovoid within said synthetic tube positioneddownstream of said forming head and having an external diametersubstantially equal to the inner diameter of said tube, said secondovoid being pierced by an opening which permits the pressure downstreamof said second ovoid to be equalized with the pressure between saidovoids,

keeping said ovoids spaced by means of a mechanical connectiontherebetw'een, utilizing a magnetic field to prevent at least one ofsaid ovoids from moving axially of said tube, and applying fluidpressure inside said tube upstream of said first ovoid.

6. Apparatus for manufacturing a flexible tubular member byhelicallycoiling a shaped metallic strip in a forming head about a tube ofsynthetic material, said apparatus comprising in combination: means foradvancing a tube of synthetic material through said forming head alongthe axis of the tubular member being formed, means defining at least oneorifice in the path of travel of said tube upstream of said forming headhaving a diameter less than the inner diameter of the tubular memberbeing formed, at least one ovoid having an outer diameter substantiallyequal to the inner diameter of the synthetic tube, and means for holdingsaid ovoid in a position inside said tube downstream of the forminghead.

7. Apparatus as claimed in claim 6 which comprises magnetic means formaintaining the ovoid in position.

8. Apparatus as claimed in claim 6 comprising a second ovoid upstream ofthe orifice, said two ovoids being connected by a duct which connectsthe spaces upstream and downstream of the two ovoids and comprising aVenturi between the two ovoids which makes it possible to creats apartial vacuum between said 0v ids b assin a fluid th ou sai duct.

8. Apti ai atus s claimed in ciaiim 6 which comprises a second ovoidpositioned upstream of said orifice, said two ovoids being connected toeach other, the upstream ovoid being in substantially sealing contactwith the upstream part of the tube, and magnetic means holding saidupstream ovoid in place, said downstream ovoid being pierced by anopening which permits the same pressure to prevail on both sides of saiddownstream ovoid.

Y o L UNITED sTATEs PATENT OFFICE CERTIFICATE OF CORRECTIGN Patent No.3,686,7M Dated August 29, 1972 Inventor) RENE. A. MAZUIR and DANIELHOFFMANN It is certified that error a ppears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

[73] Assignee: Societe Anonyme Des Hauts Fourneaux De La Chier-sLongwy-Bas, France Signed and sealed this 13th day of March 1973..

(SEAL) Attest:

EDWARD M.PLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents ORM PO-105O (10-69)

1. In a process for manufacturing a flexible tubular member by helicallycoiling a shaped metallic strip in a forming head about a cylindricaltube of synthetic material, the improvement according to which saidprocess comprises the steps of bringing the tube of synthetic materialinto alignment with the tubular member being formed, at least partiallyflattening the tube of synthetic material, and passing it through atleast one orifice having a curved periphery and a diameter less than theinternal diameter of the tubular member being manufactured, whereby thetube of flattened synthetic material is shaped within said orifice tohave an arcuate cross-section, passing said tube through the forminghead, and restoring said tube of synthetic material to its initialcylindrical shape inside the tubular member which has been formed bypassing it around an ovoid within said synthetic tube downstream of saidforming head having an external diameter substantially equal to theinner diameter of said tube, while preventing axial movement of saidovoid in said tube by exerting fluid pressure against the downstreamside of said ovoid.
 2. In a process for manufacturing a flexible tubularmember by helically coiling a shaped metallic strip in a forming headabout a cylindrical tube of synthetic material, the improvementaccording to which said process comprises the steps of bringing the tubeof synthetic material into alignment with the tubular member beingformed, at least partially flattening the tube of synthetic material,and passing it through at least one orifice having a curved peripheryand a diameter less than the internal diameter of the tubular memberbeing manufactured, whereby the tube of flattened synthetic material isshaped within said orifice to have an arcuate cross-section, passingsaid tube through the forming head, and restoring said tube of syntheticmaterial to its initial cylindrical shape inside the tubular memberwhich has been formed by passing it around an ovoid within saidsynthetic tube downstream of said forming head, said ovoid havingmagnetic properties and an external diameter substantially equal to theinner diameter of said tube, while preventing movement of said ovoidaxially of said tube by subjecting it to a magnetic field.
 3. In aprocess for manufacturing a flexible tubular member by helically coilinga shaped metallic strip in a forming head about a cylindrical tube ofsynthetic material, the improvement according to which said processcomprises the steps of bringing the tube of synthetic material intoalignment with the tubular member being formed, by passing said tube ofsynthetic material over a firsT ovoid having a diameter substantiallyequal to the diameter of said tube, at least partially flattening thetube of synthetic material, and passing it through at least one orificehaving a curved periphery and a diameter less than the internal diameterof the tubular member being manufactured, whereby the tube of flattenedsynthetic material is shaped within said orifice to have an arcuatecross-section, passing said tube through the forming head, and restoringsaid tube of synthetic material to its initial cylindrical shape insidethe tubular member which has been formed by passing it around a secondovoid within said synthetic tube positioned downstream of said forminghead and having an external diameter substantially equal to the innerdiameter of said tube, said second ovoid being connected to said firstovoid by a duct provided with a Venturi opening into said tube betweensaid ovoids, and said tube being flattened by passing a fluid throughsaid duct to cause said Venturi to reduce the pressure in said tube. 4.Process as claimed in claim 3 which comprises the step of utilizing amagnetic field to hold at least one of said ovoids in position.
 5. In aprocess for manufacturing a flexible tubular member by helically coilinga shaped metallic strip in a forming head about a cylindrical tube ofsynthetic material, the improvement according to which said processcomprises the steps of bringing the tube of synthetic material intoalignment with the tubular member being formed, by passing said tube ofsynthetic material over a first ovoid having a diameter substantiallyequal to the diameter of said tube, at least partially flattening thetube of synthetic material, and passing it through at least one orificehaving a curved periphery and a diameter less than the internal diameterof the tubular member being manufactured, whereby the tube of flattenedsynthetic material is shaped within said orifice to have an arcuatecross-section, passing said tube through the forming head, and restoringsaid tube of synthetic material to its initial cylindrical shape insidethe tubular member which has been formed by passing it around a secondovoid within said synthetic tube positioned downstream of said forminghead and having an external diameter substantially equal to the innerdiameter of said tube, said second ovoid being pierced by an openingwhich permits the pressure downstream of said second ovoid to beequalized with the pressure between said ovoids, keeping said ovoidsspaced by means of a mechanical connection therebetween, utilizing amagnetic field to prevent at least one of said ovoids from movingaxially of said tube, and applying fluid pressure inside said tubeupstream of said first ovoid.
 6. Apparatus for manufacturing a flexibletubular member by helically coiling a shaped metallic strip in a forminghead about a tube of synthetic material, said apparatus comprising incombination: means for advancing a tube of synthetic material throughsaid forming head along the axis of the tubular member being formed,means defining at least one orifice in the path of travel of said tubeupstream of said forming head having a diameter less than the innerdiameter of the tubular member being formed, at least one ovoid havingan outer diameter substantially equal to the inner diameter of thesynthetic tube, and means for holding said ovoid in a position insidesaid tube downstream of the forming head.
 7. Apparatus as claimed inclaim 6 which comprises magnetic means for maintaining the ovoid inposition.
 8. Apparatus as claimed in claim 6 comprising a second ovoidupstream of the orifice, said two ovoids being connected by a duct whichconnects the spaces upstream and downstream of the two ovoids andcomprising a Venturi between the two ovoids which makes it possible tocreats a partial vacuum between said ovoids by passing a fluid throughsaid duct.
 9. Apparatus as claimed in claim 6 which comprises a secondovoid positioned upstream of said orifice, said two ovoIds beingconnected to each other, the upstream ovoid being in substantiallysealing contact with the upstream part of the tube, and magnetic meansholding said upstream ovoid in place, said downstream ovoid beingpierced by an opening which permits the same pressure to prevail on bothsides of said downstream ovoid.